1. Field of the Invention
The present invention relates to a video signal reproducing apparatus, and more particularly to a system for recording and reproducing a video signal by rotating heads in combination with a tape-shaped recording medium, and having a function of variable-speed reproduction.
2. Related Background Art
Among known record/reproducing systems there is a digital video tape recorder (DVTR), and a picture search function with variable speed is known in such a digital video tape recorder.
In the following there will be explained, as an example, a digital video tape recorder for recording and reproducing a high-quality video signal with two sets of paired heads having a mutual phase difference of 180.degree.. FIG. 1 shows the recorded pattern on a magnetic tape 42, obtained with such recorder. The revolution of the recording heads is assumed to be 1,800 rpm.
In FIG. 1, tracks 110 and 111 record the signal of a first field of an interlaced video signal, and tracks 112 and 113 record the signal of a second field. Thus, the video signal of a field is recorded on two tracks. The track 110 is recorded by a magnetic head 38R, while the track 111 is recorded by a magnetic head 38L. The track 112 is recorded by a magnetic head 40R which has a same azimuth angle as that of the head 38R and rotates with a phase difference of 180.degree. with respect thereto, and the track 113 is recorded by a magnetic head 40L which has a same azimuth angle as that of the head 38L and rotates with a phase difference of 180.degree. with respect thereto. FIG. 1 shows a state in the course of recording of a 7th field with the magnetic heads 38R, 38L.
The track 110 records the information, among 1044 effective scanning lines of a frame, of 261 scanning lines of Nos. 000, 004, 008, 012, . . . , 1040 of a half of the first field, while the track 111 records the information of 261 scanning lines of Nos. 002, 006, 010, 014, . . . , 1042 of another half of the first field. Likewise the track 112 records the information of 261 scanning lines of Nos. 001, 005, 009, 013, . . . , 1041 of a half of the second field, and the track 113 records the information of 261 scanning lines of Nos. 003, 007, 011, . . . , 1043. FIG. 2 shows a data block of the video information of a scanning line, in which ID indicates the number of scanning line and the number of frame; CRC is an error correction code of ID; DATA is the video data of said scanning line; and C1 is an error correction code for DATA.
In the picture search operation in the conventional structure explained above, the revolution of the capstan motor is changed to increase the tape speed. As the magnetic head will be positioned over two tracks in certain positions, the data reproduction will become impossible in such positions unless so-called DTF (dynamic track following in which the rotary head is shifted in a direction of an axis thereof) is employed. If the tape speed in such search operation is selected as an integral multiple, such as twice or six times, of the standard tape speed, the failure of data reproduction always takes place in a same position in the image frame. In order to prevent such a phenomenon, the search tape speed is usually selected as a non-integral multiple of the standard tape speed.
FIG. 3 shows an example of the tracing pattern of the magnetic heads in case said search tape speed is 2.4 times the standard tape speed. There are shown tracing trajectories 120A-120B by the head 38R, 121A-121B by 38L, 122A-122B by 40R, and 123A-123B by 40L. As shown in FIG. 3, the magnetic head obliquely crosses the tracks by the change in the tape speed. When the azimuthal angle is inverted at every track, signal reproduction is possible in hatched areas in FIG. 3. It is assumed that the signal reproduction is not affected by a track of inverse azimuthal angle, and that the signal reproduction is possible as long as the positional aberration between a head and a track of the same azimuth does not exceed 40%.
Let us consider a case in which the information of a field is recorded in a track in the order from top to bottom of an image. The hatched areas 130, 131 in FIG. 3 contain information of the scanning lines 000-296 of an even field 0, while the hatched areas 132, 133 contain information of the scanning lines 745-1043 of an odd field 1. FIG. 4 is a schematic view of an inter-frame interpolated image obtained by arranging the data of thus successively reproduced scanning lines in a frame memory, wherein numerals indicate the order of fields in time, counted from an even field 0.
On the other hand, FIG. 5 is a schematic view of an image reproduced by storing data which could be reproduced in each rotation of the rotating head, in the frame memory and generating the unreproduced data by interpolation in the frame from the preceding reproducing scanning lines, wherein the numerals indicate the order of fields as in FIG. 6.
Also, FIG. 6 shows the tracing pattern in case the search tape speed is 6.8 times of the standard tape speed. FIG. 7 schematically shows an image obtained by interframe interpolation in case the data reproduction is possible within a range of an envelope from 100 to 60% of the normal reproduction, while FIG. 8 schematically shows an image obtained by interpolation within the frame. The oblique scanning angle .theta. of the magnetic head on the magnetic tape is usually about 5.degree.-6.degree.. In FIG. 6, the magnetic head appears to run inversely on the magnetic tape because said angle .theta. is represented larger for the purpose of clarity, but such inverse running does not take place in practice.
Thus, in the image of a frame reproduced in a turn of the rotating drum, each reproduced image area becomes larger, but the image area that cannot be reproduced also becomes larger when the search tape speed is close to the standard speed. On the other hand, when the search tape speed is faster, the reproduced image area and the unreproduced image area are repeated in smaller portions in the image.
In the conventional picture search operation, therefore, the interframe interpolation provides a satisfactory image in case the search tape speed is close to the standard speed, but the image gives an uncomfortable impression when the search tape speed becomes faster, because the image becomes composed of fractional images different in time.
Such an uncomfortable impression can be reduced even for a fast search tape speed, by employing, for the picture search, the interpolation within a frame and substituting the unreproduced scanning line with a reproduced scanning line closest in time, since the reproduced image portions are scattered in small vertical portions on the image. However, when the search tape speed becomes close to the standard speed, the vertical width of the unreproduced image area becomes larger on the image, and the interpolation within a frame gives an unnatural impression as an image area of significantly lower resolution moves up and down in the image.